U.S. patent application number 14/781297 was filed with the patent office on 2016-02-18 for fast magnetic resonance imaging method and system.
This patent application is currently assigned to Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. The applicant listed for this patent is Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. Invention is credited to Yiu-cho Chung, Xin Liu, Hairong Zheng, Yanjie Zhu.
Application Number | 20160047873 14/781297 |
Document ID | / |
Family ID | 51622408 |
Filed Date | 2016-02-18 |
United States Patent
Application |
20160047873 |
Kind Code |
A1 |
Chung; Yiu-cho ; et
al. |
February 18, 2016 |
FAST MAGNETIC RESONANCE IMAGING METHOD AND SYSTEM
Abstract
A fast magnetic resonance imaging method and system, the method
comprising: applying a periodic radio-frequency pulse train to
drive the magnetization vector of an imaging volume into a steady
state (102); acquiring a free induction decay (FID) signal and an
echo signal alternately in a steady-state free precession sequence
(104); conducting an FID signal imaging and a T2-weighted imaging
(106). Alternate acquisition of the FID signal and the echo signal
in the steady-state free precession sequence effectively increases
the signal-to-noise ratio (SNR) of the acquired signals and reduces
the sensitivity of the sequence to motions.
Inventors: |
Chung; Yiu-cho; (Shenzhen,
CN) ; Zhu; Yanjie; (Shenzhen, CN) ; Liu;
Xin; (Shenzhen, CN) ; Zheng; Hairong;
(Shenzhen, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shenzhen Institutes of Advanced Technology, Chinese Academy of
Sciences |
Shenzhen |
|
CN |
|
|
Assignee: |
Shenzhen Institutes of Advanced
Technology, Chinese Academy of Sciences
Shenzhen
CN
|
Family ID: |
51622408 |
Appl. No.: |
14/781297 |
Filed: |
March 29, 2013 |
PCT Filed: |
March 29, 2013 |
PCT NO: |
PCT/CN2013/073455 |
371 Date: |
September 29, 2015 |
Current U.S.
Class: |
324/309 ;
324/322 |
Current CPC
Class: |
G01R 33/5614 20130101;
G01R 33/5602 20130101; G01R 33/5608 20130101 |
International
Class: |
G01R 33/561 20060101
G01R033/561; G01R 33/56 20060101 G01R033/56 |
Claims
1. A method for fast magnetic resonance imaging, comprising:
applying a periodic radio-frequency pulse train to drive
magnetization vector of an imaging volume into a steady state;
acquiring an FID signal and an echo signal in a steady-state free
precession sequence alternately; conducting an FID signal imaging
and a T2-weighted imaging.
2. The method according to claim 1, wherein the acquiring the FID
signal and the echo signal in a steady-state free precession
sequence alternately is by removing a part of a compensation
gradient from, and/or adding a gradient crusher into a balanced
steady-state free precession sequence.
3. The method according to claim 2, said the removing the part of
the compensation gradient from the balanced steady-state free
precession sequence, comprising: removing the compensation gradient
before a next radio-frequency pulse when acquiring the FID signal;
removing the compensation gradient after a present radio-frequency
pulse when acquiring the echo signal.
4. The method according to claim 2 or 3, the removing the part of
the compensation gradient from the balanced steady-state free
precession sequence, comprising: removing the compensation gradient
along a slice selection direction and/or a phase encoding direction
from the balanced steady-state free precession sequence.
5. The method according to claim 2, the adding the gradient crusher
into the balanced steady-state free precession sequence,
comprising: adding the gradient crusher along the phase encoding
direction.
6. A system for fast magnetic resonance imaging, comprising a fast
imaging module configured to: drive magnetization vector of an
imaging volume into a steady state; acquire an FID signal and an
echo signal in a steady-state free precession sequence alternately;
and conduct an FID signal imaging and aT2-weighted imaging.
7. The system according to claim 6, wherein the fast imaging module
is further configured to remove a part of a compensation from,
and/or add a gradient crusher into a balanced steady-state free
precession sequence, to acquire the FID signal and the echo signal
in the steady-state free precession sequence alternately.
8. The system according to claim 7, the fast imaging module is
further configured to remove the compensation gradient before a
next radio-frequency pulse, when acquiring the FID; and remove the
compensation gradient after a present radio-frequency pulse when
acquiring the echo signal.
9. The system according to claim 7 or 8, the fast imaging module is
further configured to remove the compensation gradient along a
slice selection direction and/or a phase encoding direction in the
balanced steady-state free precession sequence.
10. The system according to claim 7, the fast imaging module is
further configured to add the gradient crusher along the phase
encoding direction.
Description
CROSS REFERENCE
[0001] The present application claims priority to the International
Application Serial Number PCT/CN2013/073455, filed on Mar. 29, 2013
and published as WO 2014/153775 A1, which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present application relates generally to the technology
of magnetic resonance imaging and more particularly to a method and
system for fast magnetic resonance imaging.
BACKGROUND
[0003] During the process of magnetic resonance imaging, if a
radio-frequency pulse is applied periodically, wherein de-phased
angles in each cycle are the same, a steady-state free precession
sequence may be formed. If the duration of the cycle is shorter
than the relaxation time T2, the transverse magnetization vector
may not disappear completely at the end of the cycle. It may remain
in the next cycle, forming a coherent steady-state signal. Since a
high signal-to-noise ratio (SNR) may be obtained for the coherent
steady-state signal when a short repetition time is used, the
technique of generating coherent steady-state signal is an
important branch of fast magnetic resonance imaging sequence
technology. So far, the main coherent steady-state sequences
include balanced steady-state free precession sequence (true FISP
from Siemens, FIESTA from GE), double echo steady-state sequence
(DESS of Siemens), etc., to name a few.
[0004] Within the repetition time (TR) of a coherent steady-state
free precession sequence, the transverse magnetization vector
consists of two high components, one being a transverse
magnetization vector formed by the flip of longitudinal
magnetization vector right after the excitation of the
radio-frequency pulse, denoted as S.sup.+; the other being a high
signal formed by the re-phasing of residual magnetization vector
being excited by the radio-frequency pulse with its accumulated
phase from dispersion to refocusing that exists before the next
radio-frequency pulse arrives, which is denoted as S.sup.-. A
completely symmetric gradient waveform is utilized by the true FISP
sequence to acquire a signal that is a complete superposition of
the signals S.sup.- and S.sup.-, and gives a high SNR. However, the
signal is sensitive to the magnetic field inhomogeneity. As the
repetition time (TR) increases, banding artifacts would appear. The
banding artifacts would deteriorate as magnetic field inhomogeneity
increases, resulting in inferior image quality. In the double echo
steady-state sequence, the gradient echo signals formed by the
S.sup.+ and S.sup.- within a single TR are acquired separately to
obtain an FID (sometimes called FISP-FID: Fast Imaging with Steady
Precession, Free Induction Decay) signal and an echo signal
(FISP-echo) respectively. However, within a single period of TR,
the two signals not only need to be acquired, but also need to be
separated completely. Thus, TR will be long, and the imaging time
increases, while the SNR decreases due to the increased TR.
Moreover due to the coexistence of the S.sup.+ and S.sup.- in the
same TR, the two signals need to be separated by applying a
gradient crusher appropriately, otherwise, mutual interference will
result in artifacts in the image.
SUMMARY
[0005] In the present application, one technical problem to be
solved is to provide a fast magnetic resonance imaging method with
good image quality and high SNR while overcoming deficiencies in
the prior art.
[0006] Another problem to be solved in the present application is
to provide a fast magnetic resonance imaging system based on the
above method.
[0007] The present application solves the technical problems by the
technical solutions below:
[0008] A fast magnetic resonance imaging method that comprise the
following:
[0009] Applying a periodic radio-frequency pulse train to enable
the magnetization vector of an imaging volume to reach a steady
state;
[0010] Acquiring an FID signal and an echo signal alternately
(i.e., FID in the odd TR cycles and echo signal in the even TR
cycles) in a steady-state free precession sequence;
[0011] Conducting FID signal imaging and T2-weighted imaging.
[0012] According to one embodiment of the present application,
alternate acquisition of the FID signal and the echo signal in the
steady-state free precession sequence is achieved by removing part
of a compensation gradient from, or adding a gradient crusher into
a balanced steady-state free precession sequence.
[0013] According to one embodiment of the present application,
removing the part of the compensation gradient from the balanced
steady-state free precession sequence comprises: removing the
compensation gradient before the next radio-frequency pulse when
acquiring the FID signal; and removing the compensation gradient
after the radio-frequency pulse when acquiring the echo signal.
[0014] According to one embodiment of the present application,
removing the part of the compensation gradient from the balanced
steady-state free precession sequence comprises: removing the
compensation gradient along the slice selection direction or
removing the compensation gradient along the frequency encoding
direction in the balanced steady-state free precession
sequence.
[0015] According to one embodiment of the present application, the
adding of the gradient crusher into the balanced steady-state free
precession sequence comprises adding the gradient crusher along the
phase encoding direction.
[0016] A fast magnetic resonance imaging system described here
comprises a fast imaging module, which is configured to apply a
periodic radio-frequency pulse train to drive the magnetization
vector of an imaging volume into steady state; acquire an FID
signal and an echo signal alternately from the steady-state signal;
and conduct an FID signal imaging and a T2-weighted imaging.
[0017] According to one embodiment of the present application, the
fast imaging module is further configured to remove a part of a
compensation gradient from, or add a gradient crusher into a
balanced steady-state free precession sequence, to acquire the FID
signal and the echo signal alternately in the steady-state free
precession sequence.
[0018] According to one embodiment of the present application, the
fast imaging module is further configured to remove the
compensation gradient before the next radio-frequency pulse when
acquiring the FID signal and remove the compensation gradient after
the radio-frequency pulse when acquiring the echo signal.
[0019] According to one embodiment of the present application, the
fast imaging module is further configured to remove the
compensation gradient along the slice selection direction or remove
the compensation gradient along the frequency encoding direction in
the balanced steady-state free precession sequence.
[0020] According to one embodiment of the present application, the
fast imaging module is further configured to add a gradient crusher
along the phase encoding direction.
[0021] Due to the adoption of above technical solutions, the
beneficial effects of the present application are:
[0022] (1) According to some embodiments of the present
application, due to the alternate acquisition of FID signal and
echo signal in a steady-state free precession sequence, the SNR of
the acquired induction decay signal is increased effectively.
[0023] (2) According to some embodiments of the present
application, due to the alternate acquisition of an FID signal and
an echo signal in a steady-state free precession sequence, the
obtained images are fully registered, and would not need any
registration during post processing.
[0024] (3) According to some embodiments of the present
application, the alternate acquisition of the FID signal and the
echo signal is adopted, which are similar to an induction decay
signal and a T2-weighted signal in the steady-state precession
sequence, the banding artifacts may be eliminated.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1 is a flow chart illustrating a method for fast
magnetic resonance imaging according to one embodiment of the
present application;
[0026] FIG. 2 is a time sequence diagram of the alternate
steady-state free precession sequence according to one embodiment
of the present application;
[0027] FIG. 3 is a graph showing how FISP FID signal and FISP echo
signal vary with the repetition time according to one embodiment of
the present application;
[0028] FIG. 4 is a graph showing how FISP FID signal and FISP echo
signal change with the flip angles of the radio-frequency pulse
according to one embodiment of the present application;
[0029] FIG. 5 is a time sequence diagram illustrating the balanced
steady-state free precession sequence in prior art;
[0030] FIG. 6 is a time sequence diagram illustrating the removal
of a compensation gradient along the slice selection direction
according to another embodiment of the present application;
[0031] FIG. 7 is a time sequence diagram illustrating the removal
of a compensation gradient along the frequency encoding direction
according to another embodiment of the present application;
[0032] FIG. 8 is a time sequence diagram illustrating the addition
of a gradient crusher along the phase encoding direction according
to another embodiment of the present application;
[0033] FIG. 9 is a block diagram illustrating the system
architecture for a fast magnetic resonance imaging according to one
embodiment of the present application.
DETAILED DESCRIPTION
[0034] The detailed description of the present application will be
described clearly and completely in conjunction with the exemplary
embodiments and accompanying drawings.
[0035] FIG. 1 illustrates a flow chart of the fast magnetic
resonance imaging method according to one embodiment of the present
application, which includes:
[0036] Step 102: applying a periodic radio-frequency pulse train to
drive the magnetization vector of an imaging volume into steady
state;
[0037] Step 104: acquiring an FID signal and an echo signal
alternately in a steady-state free precession sequence;
[0038] Step 106: conducting an FID signal imaging and a T2-weighted
imaging.
[0039] According to this embodiment, a periodic radio-frequency
pulse train is applied to drive the magnetization of an imaging
volume into steady state. These two signals are then acquired and
conduct an FID signal imaging and a T2-weighted imaging by
acquiring an FID signal and an echo signal alternately in a
steady-state free precession sequence. The sequence may have a
basic waveform as shown in FIG. 2 if the waveform is depicted in a
Cartesian coordinate.
[0040] As shown in the time sequence diagram of FIG. 2, by using
radio-frequency pulses with identical TR and the same de-phased
angles generated by additional gradient in every cycle, the
requirements to generate steady-state coherent signals are
satisfied. Thus, the magnetization vector may enter into a
steady-state after certain number of excitation cycles. Based on
the balanced steady-state free precession sequence, some of the
compensation gradient are removed to acquire an FID signal and an
echo signal alternately. As shown in FIG. 2, the compensation
gradient is removed before the next radio-frequency pulse when
acquiring an FID signal and the dephasing gradient is removed after
the radio-frequency pulse when acquiring an echo signal, which is
similar to the role of a gradient crusher. The gradient crusher may
also be added along the frequency encoding direction (shown as the
broken lines in FIG. 2), the phase encoding direction, or any
combination of the three directions. The axis of the gradient
crusher may affect the sensitivity of blood flow along its
direction, but the added gradient crusher need to guarantee that
the dephased angles of each pixel are more than 27t. The time in
which the gradient crusher is applied may vary with the axis where
the gradient crusher is applied along, which may affect TR, and it
should be determined according to the practical scenario in
specific applications.
[0041] It's suggested by simulations that FIG. 3 shows that TR and
flip angles are the key factors affecting intensity and contrast of
the two signals (FID and echo). The intensity of the FID signal
increases as TR increases, whereas the intensity of the echo signal
decreases as TR increases since the echo signal has a T2-weighted
component. The contrast of the sequence for tissues with different
Ti and T2 values varies with the variation of the degree of flip
angles. This imaging technology may be used for the estimation of
the parameter T2, imaging of blood flow etc. Detailed parameters
may be setup based on need in practical applications.
[0042] The acquisition of an FID signal and an echo signal
alternately in a steady-state free precession sequence disclosed
herein may be appropriate for use in two dimensional (2D) and three
dimensional (3D) imaging, and may also be applicable to various
sampling strategies such as Cartesian sampling, spiral sampling,
radial sampling or other K-space sampling methods.
[0043] FISP_FID(S.sup.+) signal and FISP echo(S) signal may be
expressed, respectively, as:
.sub.0.sup.+=.intg..sub..theta.1.sup..theta.2QE.sub.2' sin
.alpha.(1-E.sub.2exp(-i.beta.))d.beta.
S.sub.0.sup.-=.intg..sub..theta.1.sup..theta.2QE.sub.2' sin
.alpha.(exp(-i.beta.)-E.sub.2)d.beta.
Wherein,
[0044] Q = 1 - E 1 1 - E 1 cos .alpha. - ( E 1 - cos .alpha. ) E 2
2 + E 2 ( E 1 - 1 ) ( 1 + cos .alpha. ) cos .beta. ##EQU00001## E 1
= exp ( - TR / T 1 ) , E 2 = exp ( - TR / T 2 ) , E 2 ' = exp ( -
TE / T 2 ) , ##EQU00001.2##
[0045] .alpha. is the flip angle of radio-frequency pulse,
[0046] .beta. is the polarization angle of the magnetization within
TR,
[0047] The integral of .beta. is in the range of [0, 2.pi.], that
is .theta..sub.2=.theta..sub.1+2.pi..
[0048] In the simulation, when T1=1000 ms, T2=80 ms, and
.alpha.=50.degree., the curve of FISP_FID(S.sup.+) and
FISP_echo(S.sup.-) signal versus TR is as shown in FIG. 3; when
T1=1000 ms, T2=80 ms, TR=4 ms, and TE=2 ms, the curve of
FISP_FID(S.sup.+) and FISP_echo(S.sup.-) signal versus flip angles
of radio-frequency pulse is as shown in FIG. 4.
[0049] According to another embodiment of the present application
of the fast magnetic resonance imaging method, slice selection is
removed to alternately form a steady-state free precession sequence
based on the embodiment shown in FIG. 1. FIG. 5 illustrates a time
sequence diagram of balanced steady-state free precession imaging
sequence, within which all gradients are balanced, and the acquired
signal is a complete overlapping of the FID and FISP_echo signal.
FIG. 6 is a time sequence diagram in which the compensation
gradient is removed along the slice selection direction.
[0050] According to another embodiment of the present application
of the fast magnetic resonance imaging method based on the
embodiment shown in FIG. 1, the compensation gradient is removed
along the frequency encoding direction (also called the readout
direction) to form the alternative steady-state free precession
sequence as illustrated in FIG. 7.
[0051] According to another embodiment of the present application
of the fast magnetic resonance imaging method based on the
embodiment shown in FIG. 1, a gradient crusher is added along the
phase encoding direction to form the alternative steady-state free
precession sequence as illustrated in FIG. 8.
[0052] Alternative steady-state free precession sequences may be
formed by the methods of removing compensation gradients along the
slice selection direction; removing compensation gradients along
the frequency encoding direction; adding a gradient crusher along
the phase encoding direction, or any combination thereof of the two
or three methods illustrated above.
[0053] FIG. 9 is a block diagram illustrating the architecture of a
fast magnetic resonance imaging system according to one embodiment
of the present application, comprising a fast imaging module which
is configured to apply a periodic radio-frequency pulse train to
drive the magnetization of an imaging volume to steady state,
acquire an FID signal and an echo signal alternately in a
steady-state free precession sequence, and conduct an FID signal
imaging and a T2-weighted imaging.
[0054] According to one embodiment of the present application, the
fast imaging module is further configured to acquire an FID signal
and an echo signal alternately in a steady-state free precession
sequence by either removing part of the compensation gradient
alternately, or adding a gradient crusher in the balanced
steady-state free precession sequence.
[0055] According to one embodiment of the present application, the
fast imaging module is further configured to remove the
compensation gradient before the next radio-frequency pulse when
acquiring an FID signal and remove the compensation gradient after
the radio-frequency pulse when acquiring an echo signal.
[0056] According to one embodiment of the present application, the
fast imaging module is further configured to remove a compensation
gradient along the slice selection direction or remove a
compensation gradient along the frequency encoding direction within
a balanced steady-state free precession sequence.
[0057] According to one embodiment of the present application, the
fast imaging module is further configured to add a gradient crusher
along the phase encoding direction.
[0058] The above is a detailed description with reference to
specific embodiments of the present application. To a person of
ordinary skill in the art, and without departing from the spirit or
concept of the present application, various simple developments or
alternations may be made. Therefore, the present application is not
limited to the embodiments listed in this disclosure, but defined
by the appended claims.
* * * * *